1. Field of the Invention
The present invention relates to a mold-clamping control method for an injection molding machine, which method is favorably used in detecting a foreign object that is caught between a movable mold and a stationary mold during the course of a mold clamping step.
2. Description of the Related Art
Conventionally, in the field of an injection molding machine designed such that reciprocating motion produced by a drive unit composed of a servomotor and a ball-screw mechanism is transmitted to a movable platen via a toggle link mechanism, there has been known a foreign-object detection method for detecting a foreign object (e.g., a molded product) that is caught between a movable mold and a stationary mold upon movement of the movable platen in a mold closing direction (see, for example, Japanese Patent Application Laid-Open (kokai) No. 2002-172670).
The patent publication discloses a foreign-object detection method for an injection molding machine, including the steps of detecting a physical value related to a mold closing action in a monitor region during the course of a mold clamping step; and performing emergency processing associated with detection of a foreign object when a deviation between the detected physical value and a predetermined theoretical value is in excess of a threshold value. In the foreign-object detection method, a trial mold clamping operation is performed to thereby detect a maximum value of the deviation; and the maximum value is added to a preset reference value to thereby determine a threshold value to be used for foreign object detection.
In the above-mentioned foreign-object detection method (mold-clamping control method), the maximum deviation between the detected physical value and the preset theoretical value is added to the reference value to thereby determine the threshold value. Thus, the method can promptly and readily determine a fixed threshold value with high accuracy and high reliability.
However, the magnitude of a physical value related to a mold closing operation in a monitor region usually varies depending on, for example, dimensional errors and wear of mechanisms, lubrication and adjustment for maintenance, and uneven rotation of a servomotor. In the case of an automatic 24-hour operation, the magnitude of a physical value varies depending on an hour of the day because of, for example, temperature variations between day and night. Thus, when the threshold value is fixed, erroneous detection may arise as a result of variations in a physical value caused by a disturbance. Such a conventional method may cause unnecessary suspension of operation, thus involving unnecessary inspection by an operator, delay in a production schedule, and a failure to provide high consistency and high reliability in terms of mold-clamping control.
Also, when a foreign object is caught between a movable mold and a stationary mold, a physical value related to a mold closing operation is not properly detected, and a serve control system performs a feedback control such that the torque of a servomotor increases sharply. In order to cope with such a sharp increase in servomotor torque, in a monitor region where detection of a foreign object is performed, a torque limit signal is supplied to a servo circuit such that torque output does not exceed a predetermined torque limit value, thereby avoiding application of an excessive force to the molds. When the torque limit value is set too low, the drive force of the servomotor is suppressed, thereby causing inconsistent operation, such as a drop in rotational speed, with a resultant occurrence of erroneous detection. Usually, a user (operator) determines the torque limit value as appropriate in view of, for example, a product to be molded and molding conditions.
However, the user encounters difficulty in setting the torque limit value, since various factors must be considered. Particularly, since the user has a high regard for molds, he/she tends to set the torque limit value low, potentially resulting in inconsistent operation and a tendency toward occurrence of erroneous detection.